Abstract
GreA is a well-characterized transcriptional factor that acts primarily by rescuing stalled RNA polymerase complexes, but has also been shown to be the major transcriptional fidelity and proofreading factor, while it inhibits DNA break repair. Regulation of greA gene expression itself is still not well understood. So far, it has been shown that its expression is driven by two overlapping promoters and that greA leader encodes a small RNA (GraL) that is acting in trans on nudE mRNA. It has been also shown that GreA autoinhibits its own expression in vivo. Here, we decided to investigate the inner workings of this autoregulatory loop. Transcriptional fusions with lacZ reporter carrying different modifications (made both to the greA promoter and leader regions) were made to pinpoint the sequences responsible for this autoregulation, while GraL levels were also monitored. Our data indicate that GreA mediated regulation of its own gene expression is dependent on GraL acting in cis (a rare example of dual-action sRNA), rather than on the promoter region. However, a yet unidentified, additional factor seems to participate in this regulation as well. Overall, the GreA/GraL regulatory loop seems to have unique but hard to classify properties.
Highlights
Gene expression regulation can take place at several levels, e.g., at the transcriptional, post-transcriptional, and translational level
Our data indicate that GreA mediated regulation of its own gene expression is dependent on greA leader encodes a small RNA (GraL) acting in cis, rather than on the promoter region
Transcription could be regulated at the stage of elongation (e.g., by the Escherichia coli GreA and GreB proteins that directly interact with RNA polymerase (RNAP), see below), or at the stage of termination (e.g., by antitermination factors, such as the Nus factors (NusA, NusB, NusG and NusE), which form a complex interaction with both RNAP and specific RNA sequences [1])
Summary
Gene expression regulation can take place at several levels, e.g., at the transcriptional, post-transcriptional, and translational level. Transcriptional regulation usually occurs at the stage of initiation, i.e., activators or repressors bind to a specific promoter region sequence, and enhance or inhibit transcription, respectively. Other examples include attenuation mediated by ribosomes or regulation by RNA aptamers bound to RNAP [2,3,4]. A large group of transcriptional regulators includes small RNA (sRNA). These short RNA species, ranging in size from 50 to 500 nucleotides (nt) [5], were shown to either act in cis (e.g., antisense RNAs [5] and riboswitches [3]) or in trans, where they enhance or inhibit translation of their targets by diverse mechanisms, relying on complementary base-pairing [6]. Trans acting sRNAs often require a protein chaperone mediating sRNA-target mRNA interaction, called Hfq [5]
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